1. Field of the Invention
This invention relates to a silicon controlled rectifier lamp supply circuit for controlling the amount of voltage delivered to illuminate lamps in a document image capturing system.
2. Description of the Prior Art
In the past, voltage supplies for illumination lamps were able to maintain an output voltage within 1-2 volts of a predetermined voltage level. Such deviations were acceptable for microfilm documentation of information, but systems using charge coupled devices (CCD) for registering document reflectances require more stable voltage supplies to maintain uniform illumination.
Prior systems recovered to their predetermined voltage levels after changes in load voltage demands within approximately 100 milliseconds. However, such a time lag with newer CCD sensitivities results in changing illumination levels which adversely affect the accuracy of the information sensing elements. Therefore, the invention provides for a faster response time of 30 milliseconds so light output changes are minimized.
In the patent by Raymond Becky entitled, "Controlled Apparatus for Silicon Controlled Rectifiers," U.S. Pat. No. 3,836,839 issued Sept. 17, 1974, the circuit is subject to instability and its transfer function may become nonlinear due to the inclusion of an adjustable feedback potentiometer at the circuit's output. Setting a reference voltage to which the output voltage is regulated involves applying an input voltage to the circuit and adjusting the potentiometer. The applicant's circuit, however, only requires the input of a predetermined voltage to the positive terminal of an operational amplifier to establish a reference voltage.
The advantages gained from the applicant's invention over the patented circuit are precision supply voltage regulation to the illumination lamps, increased operational stability since fixed components in a closed, negative feedback loop need not be continually adjusted for a chosen reference voltage, and a faster response to changing load voltage demands.